By Russ Britt

Investors dropped their shares of Ariad Pharmaceuticals like a bag of rocks Wednesday after the company said it was putting a hold on new patient enrollment for tests on its leukemia drug Iclusig. The reason? A higher number of patients than expected experienced serious circulatory blockages within two years after taking the medication.

Ariad
/quotes/zigman/57221/quotes/nls/ariaARIA shares plummeted more than 70% to $4.93, losing $12.21 apiece. Ariad said it was working with the U.S. Food and Drug Administration on how to handle the events in Iclusig testing.

The company said after two years of follow-up research on patients taking the drug, 11.8% suffered serious arterial thrombosis, or blood clots. Of those, 6.2% experienced cardiovascular trouble, another 4% suffered vascular issues of the brain and 3.6% had peripheral vascular trouble. Some patients had more than one type of issue, the company said. After 11 months of follow-up, the percentage of those experiencing similar issues stood at 8%.

Another 2.9% experienced serious venous occlusion, or vein blockages, after 24 months, compared with earlier findings of 2.2%. And a total of 20% of the test subjects experienced either serious or non-serious artery and vein troubles after receiving treatment.

Ariad said in a press release that testing would continue on patients in clinical trials, but future patients will be screened to exclude those who have experienced blood clots severe enough in the past to result in a heart attack or stroke.

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Health Exchange guides investors to the crucial market intelligence they need to keep up with the health care industry, which makes up one-sixth of the U.S. economy. Anchored by Russ Britt, Health Exchange is the essential site for those looking for the most important news, data and analysis on the sector. You can reach Russ at Rbritt@marketwatch.com.